Cathode-ray tube



Dec. 23, 1958 J. HAANTJES ETAL 2,865,129

CATHODE-RAY TUBE Filed Feb. 27, 1956 4 Sheets-Sheet 1 INVENTOR JOHAN HAANTJES GERRIT JAN LUBBEN AGE T Dec. 23, 1958 J. HAANTJES ETAL 2,866,129

CATHODE-RAY TUBE Filed Feb. 27, 1956 4 Sheets-Sheet 2 INVENTOR JOHAN HAANTJ ES GERRITJAN LUBBEN AGENT Dec. 23, 1958 J- HAANTJES ETAL 2,866,129

CATHODE-RAY TUBE 4 Sheets-Sheet 3 Filed Feb. 27, 1956 s x mm m N N HJ e E mnfiA W 5 m Y o B Dec. 23, 1958 J. HAANTJES ETAL 2,866,129

CATHODE-RAY TUBE Filed Feb. 27, 1956 4 Sheets-Sheet 4 INVENTOR JOHAN HAANTJES GERRIT JAN LUBBEN BY %Mi//%7 AGENT CATHODE-RAY TUBE Johan Haantjes and Gerrit Jan Lubben, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application February 27, 1956, Serial No. 568,101

Claims priority, application Netherlands March 5, 1955 8 Claims. (Cl. 315-24) The invention relates to cathode-ray tubes provided with a substantially flat screen arranged substantially at right angles to the axis of the tube and provided with a deflector coil system for deflection in two directions substantially at right angles to one another, in which tubes the electron beam, after passing through a concentration or focusing field and on reaching the field of the deflector coil system, has a substantially circular crosssection, the size of the light spot of the electron beam on the screen being substantially constant in one of the directions of deflection throughout the entire scanned area of the screen.

For colour television systems cathode-ray tubes are known in which the image. screen of the tube consists of vertical strips of material luminescent in different colours so that in the case of horizontal scanning, for example, a red strip, a green strip and a blue strip are struck in succession. When a red strip is struck, an electric signal voltage which corresponds to the red component of the image to be reproduced is simultaneously supplied to a control electrode of the cathode-ray tube. In this system it is necessary that the size of the light spot measured in the horizontal direction does not exceed the width of the strips and this requirement must be satisfied at all points of the screen area to be scanned. If for example, in a non-deflected position of the electron beam, that is to say centrally of the screen, the light spot has a substantially circular cross-section and, in addition, if the diameter of said circle is less than the width of the strips, with a high degree of deflection of the electron beam by means of the usual deflector coil systems, the shape of the light spot will vary due to astigmatism and image field curvature so that the size in the horizontal direction will exceed the width of the strips. Thus, in a cathode-ray tube, astigmatism causes a focus defect in which the electrons in diflerent axial planes come to focus at diflerent points. For one direction of deflection, the bundle of rays forms a line extending perpendicular to the said one direction of deflection. This line is called the primary or meridional focus, and the plane perpendicular to the tube axis in which the meridional focus is located is called the meridional image plane. For the same direction of deflection, the bundle of rays also forms a line extending parallel to the said one direction of deflection. This line is at a greater image distance than the first line and is called the secondary or sagittal focus, and the plane in which the latter is located is called the sagittal image plane. At all other image distances, the bundle of rays forms an ellipse or a circle. The same is true of the other direction of deflection, except that the sagittal image plane is closer to the electron source than the meridional image plane. In order to avoid this inconvenient phenomenon additional steps have to be taken.

Cathode-ray tubes are also known in which the image screen comprises horizontal strips, in which system it is desirable for the size of the light spot in the vertical direction to be less than the width of the strips, so that es i. atent ice the same difliculties arise as in the above-mentioned system.

It is an object of the invention to provide a cathoderay tube provided with such a deflector coil system that without further expedients the size of the light spot on the screen in one of the directions of deflection is substantially constant throughout the entire scanned surface area.

The invention is characterized in that for said direction of deflection the meridional image area substantially coincides with the screen surface and that for the other direction of deflection the sagittal image plane substantially coincides with the screen surface a It should be noted that it has already been proposed to use a deflector coil system in which for one direction of deflection the meridional image plane substantially coincides with the screen surface and for the other di rection of deflection the sagittal image plane substantially coincides with the screen surface. However, this deflector coil system was used in combination with a cathode-ray tube the electrons of which, after passing through a focussing field and on reaching the field of the deflector coil system, are concentrated within a ribhon-shaped part of the tube space. Consequently, in such a tube use was made either of a ribbon-shaped beam or, for example, of three beams situated in one plane. This prior combination permitted of producing a substantially point image on the fiat tube screen in spite of a large transverse dimension of the beam in one of the directions of deflection.

In contradistinction thereto, the present invention relates to the combination of such a deflecting system and a cathode-ray tube having an electron beam which on reach ing said deflecting system has a substantially circular cross-section. It is based on the following theory. When using a tube having a ribbon-shaped electron beam, so that in a plane at right angles to the electron beam said beam has a substantially rectangular cross-section, one side of the rectangle being very small and the other side being comparatively large, by means of the above-mentioned deflecting system a substantially point image is produced at each point of the screen. If, now, the larger side of the rectangle which forms the cross-section of the ribbon-shaped electron beam is reduced, this image remains the same. However, if the smaller side of the rectangle is simultaneously increased so that the cross-section of the beam becomes a square, which in practice will, however, be shaped in a substantially circular form, in the direction parallel to the initially larger transverse dimension of the beam the size of the light spot will remain the same throughout the screen, but in the direction at right angles thereto the size of the light spot will be increased. By means of the said deflecting system and of a cathode-ray tube having a substantially circular electron beam a light spot is thus produced the size of which, measured in one of the directions of deflection, is substantially constant at any point of the screen to be scanned.

The invention will be described in detail with reference to the accompanying drawings, in which:

Figs. 1 and 2 illustrate the occurrence of image defects and the reduction of these defects in a cathode-ray tube having three beams,

Fig. 3 shows a cathode-ray tube having a deflecting system in accordance with a part of an embodiment in accordance with the invention for deflection in one direction,

Fig. 4 shows the associated system for deflection in the second direction,

Fig. 5 is a developed view of the coil shown in Fig. 3 in a plane,

Fig. 6 a developed view of the coil shown in Fig. 4 in a plane,

e enaa Fig. 7.illustrates the quantities occurring with the coils H Fig. 1'1, an embodiment of a circuit arrangement of the coil components shown in Fig. 8 and, Fig. 12 a secondembodiment .of such a circuit arrangement.

Fig. 1 shows. co-ordinate axes x, if, z at right angles to one another the origin of which coincides with the deflection centre of a deflector coil system (not shown), which centre is assumed to be concentrated in one point. This coil system provides "theidefiection inthe x and ii directions of a beam which, in the non-'deflectedcondition, is. concentrated so as to surround the axis of the tube, in the present case' the z axis, and the electrons of which move in .the direction of the positive z'axis. The beam is produced by three electron beams supplied by three electron guns 1, 2 and 3 which are arranged symmetrically with respect to the z axis. The beam diverges until it reaches the focussing field the influence of which is as.- sumed to be concentrated in a plane at right angles to the z axis which. passes through the point 5. 1 After pass.- ing through. the =focussing field the beam converges and passes through the deflecting system at o. The beam diameter occurring at o determinesthe .value of the defects produced. it there is no deflection, the beam converges in the point 6 of the z axis. 'The substantially flat screen of the cathode-ray tube is arranged at right angles .to the z axis and passes through the point 6. Of this screen only the line of intersection with the plane x=0 is shown.

In addition, the figure shows what will happen without further expedients when the beam is deflected in the negative if direction in the direction of the line 8 in the plane x=o. Reckoned from the origin 0 there will be :produced, in succession, a linear cross-section of the beam 9 situated in the plane x,=0, an elliptical crosssection 10 the major axis of which extends parallel to the 'line 9, a circular cross-section 11, another-elliptical crosssection 12 the major axis of which is at rightangles to the direction of the linear cross-section 9 and finally another linear cross-section 13 which is at right-angles to the linear cross-section 9 and consequently atright angles to the plane x.=0. i

The linear cross-section 9 is situated in an image plane -of which the line 14 intersecting the plane -x=o is shown only. This image plane 14 is the sagittal image plane *for the deflection in the ij'direction. The term direction of deflection and associated sagittal image plane as used herein means the image plane in which a linear crossssectionof the beam is produced which linear'cross-section ,extends so as to be parallel to the direction of'deflection concerned. t 1

The ,-.circular cross-section 11 is situated in the plane of the mean image field curvature, of which plane the line 15 intersecting the plane x=0 is shown only.

The linear cross-section 13 is situated in the meridional image plane of which the line 16 intersecting the plane x=0 isv shown only. The term direction of deflection and associated meridional image plane asused herein is to'beunderstood to mean the image plane in which a linear vcross-section ofthe'beam is produced, which :linearcrosssection is at right concerned. 1

For the sake of completeness it should be noted that, although the term image is :used this does' notmean that, for example, one definite point of the sagittalimage ;9 corresponds to ,only one definite, "point {of thegbe'azn cross-section. In addition, in the definitions IQffilCflCCtlS angles to the direction of deflection made to a linear cross-section in the meridional or sagittal planes, but this does not mean that this linear'cross-section cannot be reduced substantially to a point.

From the above it will be seen that, if no special steps are taken, the beam, which in the non-deflected condition converges in a point 6 on the screen 7, in the deflected condition does not converge in any one point of the screen. Howeven-i'f the beam cross-section is not large, the circular cross-section 11 will be very small in the plane of the mean image field curvature. Furthermore, it is known that with a proper choice of the shape of the deflector coil systems the sagittal image plane 14 and the meridional image plane 16 will substantially coincide for both directions of deflection with the image plane 15 of the mean image field curvature. Thus, a beam having a comparatively small diameter is substantially concentrated into a point inth'e plane 15, However, this plane 1l5 is -a plane having a finite radius of cprvatnre so that it does not coincide with the plane of the substantially fiat screen 7, with the resultthat on this screen 7 no substantially point image is produced. Therefore it is usual to control the .value of the concentrating field at 5 as'a function ofthedeflection in the x and 1'] directions.

The deflecting ,field H of the deflector coil system can be determined by three .components at right angles to one anotherfidx, i. z), bs f, z) n Hair/ii. 2), which threecomponentsrnaybe determined by x,,ij and z as shown. This field .mustsatisfy the following conditions with respect to symmetry:

. h eHO H2 an H4 are fi nt hi pend z .on yn a e mean a d e et q thz e t Terms containing H; are not of importance for the present discu s o w ich a e res cte t v a .1 Q e th d orderhe .c mpcnent Q -the efi t in th re.t o v si fla sha whi h-isras r t i b in er ang 4x an i t h r eh and terms 9 the eavetions.

'-Inthe plane x=o, for the x component of the field strength it is found that HA ij, z).=H,,+H if By measuring the field in the x direction in the plane -,x.=o the values H, and H ;can be determined, and-it .will be found that these values vary as a function of z. It should be noted that the sign of H determines the signof the direction of deflection. Hereinafter the sign of H is to be understood to mean the sign produced when H is positive.

The paths of the electronscan be calculated with the aid ofFermatsprinciple.which-says that the paths which the electrons take betweentwo points are those paths .between saidpoints forwhichthe time taken to traverse them is a minimum. From this the pathsat smallvalues of the deflection can ber cal-culated. .When calculating for predetermined-'1 initial conditions, that is to ;s ayithe oint ixb,yijo, z -attwhichdhebeam reaches; the. deflection-field .imthei direction .x'5, .ij. which. point consequently-is. situated at the end of the coil spaced away from the screen, by the greatest distance, in Fig. 1, between the concentrating field and the plane 2::0, it is found that the deflection II in the i1 direction is independent from the initial conditions, that is to say 2 IJ=-Kf dz H dz The term deflection is to be understood to mean the co-ordinate of the electron in a plane z=constant, z=zs being the plane 7 ofthe screen in which the deflection U is produced. Consequently, if without deflection the electrons strike the plane 2:2, in the point 6, they continue to do so as a first approximation at small values of the deflection. In future formulas, the subscript s refers to points located at or values determined at the planar screen 7.

For the sake of completeness it should be noted that K is a constant.

When the calculation is extended to include the terms in the expressions for H H and H which are quadratic in x, ij and x, ij, deflections are found which differ from the deflection IJ found as a first approximation, which deflection for the plane z=Z is now referred to at U and similarly for the deflection in the x direction deviations from the deflection X =X found as a first approximation.

When the undeflected beam strikes the point X =0, 1],:0, the values of these'deviations Ax and Aij are where x, and ii, are as a first approximation the directional coefficients at the point x=0, ij=0, z=z, and the various coefficients a and B are integral functions of H and H The terms which are in linear relationship with the angle of aperture cause the defects of astigmatism and image field curvature. The defects which vary with the square of angle of aperture produce the coma, while the distortion is independent from the angle of aperture.

Consequently, in respect of astigmatism and image field curvature:

or, for better recognition, in polar coordinates:

where r is the distance from the z axis and (p .the angle with the x axis of the radius vector of the point x ij z situated in front of the deflecting field. Elimination of produces an ellipse having the half axes I i that of the meridional plane 1 E; r and that of the plane of the mean image field curvature vi-I 2 The image is referred to as anastigmatic when these planes coincide, that is to say when a =fi Now If, now, it is required that a =fi2, this can be eifected by a proper choice of the value and the sign of H However, the image field curvature, which is determined by cannot be removed, since in this case it is also required that a +fi =cu In this sum, the integral comprising H is no longer included and the other turns together are always positive.

From this it follows that it is not possible to construct a deflector coil system which causes a beam which converges in the non'defiected condition to convert invariably after deflection in a plane, when the cross-section of the beam may be shaped in any form.

A first recognition on which the prior suggestion is based is a choice of the beam cross-section which permits a flat'plane of convergence to be produced with respect to astigmatism and coma. For this purpose a ribbon-shaped beam is used. In a three-colour television tube the three electron guns are, for example, arranged in line with one another, which line coincides with one of the directions of deflection. Thus, the three beams together produce one ribbon-shaped beam.

If the ribbon shape is arranged in the if direction, as is shown in Fig. 2, in which corresponding elements are designated similarly to those shown in Fig. 1, for all points of the beam (p is substantially equal to so that in the plane z=z the deviation Ax deduced hereinbefore has invariably zero value. As a result the length of the meridional image 13 shown in Fig. l, which image is situated slightly in front of the plane z=z,, in the image 13 shown in Fig. 2 is substantially reduced to zero. In order to obtain a flat plane of convergence it is only necessary that the meridional image plane becomes a flat plane, that is to say that 18,:0. It should be noted that the very slight length which the meridional image 13 still exhibits is determined by the width of the ribbon-shaped beam.

If the ribbon shape were chosen in the x direction, with deflection in the ij direction it must be ensured that 11 :0, which means a fiat sagittal image plane.

Consequently, for the deflector coil which ensures the deflection in the direction parallel to the largest transverse dimension of the ribbon-shaped beam, the meridional image plane must become a flat plane, while for the deflection coil which ensures the deflection in a direction at right angles to the largest transverse dimension the sagittal image plane must become a flat plane.

In the expression 2 for 13 the first integral is positive so that the second integral must be negative in order to ensure that 3 :0. This means that H must be predominantly positive throughout the length of the coil, since the deflection II is negative with a positive field in the x direction. The variation of H: as a function of 2 may be comparatively arbitrary aslong as -the value of'the integral remains the same. The largest contribution to the integral is provided by the values of H at a high value of z, that is to say at the end of the coil system adjacent the screen, since both I] and (z'2:,). varyas the-square of z. Consequently, at this end of the coil the mean value of H must be positive.

For the deflector coil system for the other direction of deflection, in the case of Fig. 2 the x direction, the m must be made .=o. From analogous considerations it follows that in this case it is necessary that for this dcflector system the quantity H at the end of the coils adjacent the screen must be predominantly negative.

When the entire deflector system fulfils the requirements made so far, with deflection in the x direction no perceptible astigmatism will occur neither will perceptible astigmatism occur with deflection in the if direction. In addition, it should-be noted that now a flat image plane has been produced so that it is no longer necessary to vary the strength of the focussing field as a function of the deflection amplitude.

The present invention is based on recognition of the fact that such a deflecting system can also be used to advantage, when the tube contains one electron beam only of substantially circular cross-section, which beam is produced, for example, by the gun 2 in Fig. 2,'whilst the size of the light spot on the screen must be substantially constant in one of the directions of deflection. When the tube is provided with the gun 2 only and the electron beam is given a greater dimension in ii direction than is indicated in Fig. 2, this does not influence the size in the ij direction of the light spot of the screen 7. If, in addition, the electron beam is given a greater diameter in the x direction, this influences on the screen 7 the dimension of the. light spot in the x direction only, so that the dimension in the x direction is increased.

It should be noted that in the prior suggestion, in which a ribbon-shaped beam or three beams were used, steps for reducing the coma defect are described. The coma is proportional to the angle of deflection and to the square of the beam diameter, which was large in the if direction. In the present invention the beam di-, ameter is much smaller so that the coma defect is slight and imperceptible, so that measures to correct it can be omitted.

So far the two deflections have been described separately. However, in practice deflections in both directions are efiected simultaneously. It is found that the expressions for the differences Ax and Aij, considered with respect to a ribbon-shaped beam, become slightly more elaborate. I

The astigmatism is then given bythe equations:

Here X and II are the first-order deflections in the plane 1:2,, in the x and ii directions.

The A coefficients are integral functions of the quantities H 11 and H (that is to say H and H of the -say: p=9.0, the astigmatism is given by:.

beam in the horizontal direction (x direction).

and

However, 13., and A are the identical integral functions as [3 and ot respectively, so that, when the value of B and A becomes zero or is reduced, the quantity H must have the same-signs and values asbefore.

The only additional requirement produced due to the simultaneous deflection in two directions is:

Now,

Wit the usu lnroper iqn o e c th y eams and the deflector coil Systems, the first three integrals together invariably provide a negative contribution, The two last terms which contain the H and, the H consequently must provide a positive contribution together. In the said latter two integrals the values of H at the end of thecoil adjacent the screen are again most significant. It is again necessary that H at said end of the coil be negative and H 1 be positive. Consequently, the first of the two integrals becomes positive (since the. deflection I] is negative with a positive field in the x direction) and the second integral, inclusive of the minus sign, becomes negative. If the sum of the two integrals is to be positive, at the end concerned of the coil, that is to say in the proximity of the screen. 1:2,, the absolute value of H must exceed the absolute value of H The cathode-ray tube 21 shown in Fig. 3 contains a single gun, which is known per se and is shown diagrammatically only, for the production of an electron beam 23. The image screen 25 of the tube is provided with parallel strips 22 made of luminescent material which, for example, cyclically reproduce different colours and extend parallel to the x direction.

The electron beam on its way to the image screen 25 passes through thefield of a focussing coil 26, which is known per se and is shown diagrammatically only, before it reaches the deflector coil system. This deflector coil system comprises the pair of coil halves 27a and 27b for the deflection of the beam in the vertical direction (ij direction) and the second pair of coil halves 28a and 28b shown in Fig.4 for the deflection of the These two pairs of coil halves are combined in the usual manher to form a single deflecting system, however, they are shown here separately for the sake of clearness.

The shape of the coil halves 27a and 27b is so chosen that the quantity In Fig. 7 the significance of the angle 1/ and the radius R is described in detail with reference to a sectional view of an arbitrary deflector coil in a plane at right angles to the axis of a tube.

One coil half comprises the conductors 29a and 29b which usually are made up from a plurality of wires insulated from one another. The other coil half comprises the conductors 30a and 30b. The broken lines indicate that on the one hand the conductors a and 20b are interconnected at the front side and the rear side of the coil and on the other the conductor 30a is connected to the conductor 3%. It will be seen from the figure that the coil does not embrace the tube completely and that the part which is not embraced is an angle of value 4 1/.

When the tube neck is a right circular cylinder and the conductors engage this neck or a surface which is concentric with this neck, the distance R by which the centre of each conductor is spaced away from the axis of the tube neck is constant throughout the length of the coil. This is the case in the coils shown in Figs. 3 and 4. However, due to the asymmetric shape the value of the angle o varies so that it has a different value for cross-sections of the coil at different points.

For the coil halves 27a and 27b the quantity h must be positive at the end 31, which implies that the angle \p at this area is less than 30.

For the coil halves 28a and 28b h must be negative at the end 32,'that is at the side of the screen also, so that for these coil halves the angle 11 at this area is more than In addition, the absolute value of h for the coil halves 28a and 2817 at the point 32 must exceed the absolute value of h for the coil halves 27a and 27b at the point 21.

When the coils are supported fro-m a surface which is a right circular cylinder, as is assumed here, the coil half 27a, for example, when developed is shaped in the form shown in Fig. 5 and the coil half 28a is shaped in the form shown in Fig. 6.

The broken lines in Figs. 5 and 6 indicate the position which the conductors of the coil halves which are operative for the deflection are required to have if h throughout their length of the coil invariably has Zero value, so that the said lines correspond to an angle ,0 of 30.

If the two coils 27a, 27b and 28a, 28b are arranged on concentric surfaces, from the requirement that the absolute value of h for the coil 28a, 2812 at the end concerned must exceed that for the coil 27a, 27b it'follows that the coil halves 28a and 28b, when developed, must be trapezoidal to a higher extent than the coil halves 27a and 27b.

If the coils do not surround a circular cylindrical part of the tube neck but are arranged entirely or in part on a conical surface, in the quantity -3+4 cos \l/ both the angle 30 in the direction of length of the coil and the radius R are changed. In this event also the above-mentioned requirements have to be satisfied.

By means of a cathode-ray tube provided with a defiector coil system as described hereinbefore the dimension of the light spot of the screen 25 in the if direction is maintained constant so as to be less than the width of the strips 22. In the x direction, that is to say in the direction of length of the strips 22, the dimension of the light spot will be slightly dependent upon the angle of deflection, owing to astigmatism.

For the sake of completeness it should be noted that the shape of the coil halves is determined by the radius R of the coil half, the length z -z,, of the coil half and the distance 2 If, now, the angle 0 designated in Fig. 7 is denoted #1 for the coil half shown in Fig. 5 at the point 2 and at the point z and for the coil half shown in Fig.

1G 6 at the point 2 1/ and at the point 2 1/ with the usual proportions:

With a radius of the coil of 3 cms., a coil length of 12.5 cms. and a distance z =44 cms., the value of yl =34.5, of =27.5, of 1p =l1.5 and of \I/4=36.5.

It should be pointed out that the coil halves have a finite thickness at the ends also, so that 2 and 2 correspond to the position of the centre of the conductors.

Fig. 8 shows an alternative embodiment of the deflector coil system. It is again assumed that the direction of length of the strips 22 of the tube screen extends in the 7: direction.

Two annular members 33 and 34 made of ferromagnetic material are arranged to surround the neck of the cathode-ray tube (not shown), which members with respect to the tube axis are arranged one behind the other. A number of coils are provided toroidally on the annular members. The deflector coil system for the deflection in the x direction comprises a first group of four coils provided on the member 33 and a second group of four coils provided on the member 34. Since Figure 8 is a side elevation from the x direction, in the figure only two coils 35 and 36 of the first group provided on the member 33 are vvisible. These coils are also shown in Fig. 9 which is a side elevation of the member 33 from the z direction. In addition, Fig. 9 shows the position of the other two coils 37 and 38 of the first group.

In Fig. 8 also the coils 39 and 40 only of the second group of four coils provided on the member 34 are visible.

Fig. 9 is a side elevation of the member 34 in the z direction in which the remainder of the coils, that is to say 41 and 42, of the second group of four also are visible. Figs. 9 and 9 also show the beam supplied by the gun 23. The coils are coupled to one another in a manner to be described more fully hereinafter. Hence, a current flows through the coils 35, 36, 37 and 38 provided on the member 33, the direction of which current is indicated in each coil in Figs. 9 and 9 internally of the member by a cross when the currents at this point flows in the direction of the positive z axis and by a point when the current at this point flows in the direction of the negative z axis.

Thus, the current flows in opposite directions through two diametrically opposed coils 36 and 37 or 35 and 38 or 39 and 42 or 40 and 41. On the member 33, such diametrically opposed coils are arranged in planes which enclose an angle of less than 30, in the present case of 25, with the plane z'j:o and on the member 34 in planes which are at an angle exceeding 30", in the present case of 43, to the plane ij=0. The quantity H of a group of four of these coils is again determined by the quantity 3+4 cos 11/ T Where t represents one half of the acute angle enclosed by two coils having the same direction of current flow, for example is equal to 35 in Fig. 9. Since a coil is wound toroidally and the major axis of the coil consequently extends substantially parallel to the z axis, the angle ,0, in contra-distinction to the angle produced by the trapezoidal saddle coils shown in Figs. 3 to 7, is constant and hence not a function of z. It, now, t;/=30", 11:0 and consequently 11 :0. If the angle is less than 30 H is positive.

The complete deflector coil for the deflection in the x direction thus comprises a part more remote from the screen, that is to say the angular member 33 pro- 1 1 vided with the coils 35', 36, 37 and 3.8, in which H is positive, and a part adjacent the screen, that isto say the annular member 34 provided with the coils 39, 4 B, 41 and 42, in which H is negative.

On the same members 33 and 34 the coil system for the deflection in the if direction is also provided. On the member 33 the four coils 45, 46, 47 and 48 are provided, of whichin Fig. 8 only two, 45- and 46, are visible and shown by broken lines in order to avoid their being mistaken'for the other coil system.

Fig. 10 is another side elevation in the z direction of themember 33 and Fig. 10 of the member 34. The member 34 is provided with four coils 49, 5t 51 and 52.

The quantity H of the parts of this latter coil system is again determined by the expression h 3 4 cos Sincethe angle 1,0. is one half of the angle enclosed by two coils having'the same current flow direction, in view of the directions of current flow shown again in Figs. 10 and 10* this angle now corresponds to 33 in Fig. 10 and to 20 in Fig. 10.

The part of. the deflector coil provided on the annular member 33 consequently has a H -value which is negative and the part of the coil provided on the member 34 has a positive l-l -value.

Thus, the. requirement is satisfied that the deflecting system for the deflection. at right angles to the direction of the strips, that is to say the if direction (coil system of Figs. 10% and 10 at the end adjacent the image screen has a predominantly positive H -value, whilst the other deflecting system, that is to say the system for the x direction (coil system of Figs. 9 and 9 at the end adjacent the image screen has a predominantly negative H value.

These H -values can be changed by changing the angles concerned. in addition, the i-l -value can be controlled by a suitable choice of the number of ampereturns on the member 33 and the number of ampere-turns of the member 3.4. in each coil group. This method also permits of ensuring that the absolute value H at the screen end of the coil system for the deflection in the x direction exceeds the absolute value of H at the screen end of the system for the deflection in the ij direction.

In a practical embodiment with the coil system shown in Figs. 8, 9 9 10* and 10 having the angles 1, shown in these figures, an image was obtained which exhibited no perceptible astigmatism in the ij direction with the following values.

The. number of ampere-turns of the coils 45, 46, 47, 48 and 49, 50, 51, 52 was in the ratio 8:5; the ratio of the number of ampere-turns of the coils 35, 36, 3,7, 3.8, to that of the coils 39., 4t), 41 and 42 was :2. The diameter of the coils, that is to say of the central layer of the annular members, was 70 mms. The total length of the coil system, that is to say from the point 53 to the point 54 in Fig. 8, was 60 mms. and the distance from the centre of the coil system to the screen was 350 mms.

Fig. 11 shows a possible method of supplying the coils of the system for the deflection in the ij direction. In this method, the coils 45, 46, 47 and 48 provided on the member 33. are connected in series, care obviously being taken to provide a connection of each part such that the direction of flow in each coil has the correct polarity in the direction of the z axis. The coils 49, 50, 51 and 52 provided on the member 34 are also series connected. Each series connection, in addition, comprises one half of a coil 55 pror'ided with a movable core 56. The series connections thus produced are connected in parallel between the supply terminals 57 and 58 to which a sawtooth current is supplied. Displacement of the core 56 permits. of varying the distribution. of current to.v the two parallel branches, the numbersf of ampere-turns for the parts of the system provided on the member 33 and. of

the system provided on the member 3.4 being varied rela tively to one another. The. coil 55 is so. arranged that it does not influence the deflection of the beam.

In the circuit arrangement shown in Fig. 12, the coils 45, 46', :47 and 48am, included. in one. arm of a bridge circuit,.the coils 49, 50,, 51 and 52 in a second arm. The third arm consists of one half of a coil 59 and the fourth arm of the other half of the coil 59, which coil is again provided with an adjustable core 60. A diagonal 63 of the bridge is connected between the centre of the coil 59 and the junction of the two groups, of four coils each. The other diagonal has input terminals 61 and 62 for the supply of the saw-tooth current connected to it. When the bridge circuit is balanced, which condition can be achieved by adjustment of the core 6%, the diagonal 63 does not pass current and an equal currentflows through both groups of coils. If the balance of the bridge is upset, more current flows through one group of four coils than through the other group. In this arrangement also, the coil 59 is arranged so that it does not act upon the deflection of the beam.

What is claimed is:

l. A cathode-ray tube comprising a substantially planar screen located substantially perpendicular to the axis of said tube; electron beam producing means located within said tube; focusing means for focusing the electron beam produced by said beam producing means; and a deflectorcoil system for deflecting the electron beam in two directions substantially perpendicular to each other, said; beam producing means and said focusing means ccoperating to producean electron beam having a substantially circular cross-section in the field of said coil system, said coil system being operable to cause said electron beam to form a linearly elongated cross-section whose longest dimension is perpendicular to one of said directions of deflection and which is located in the meridianal image plane which substantially coincides with the plane of said screen for said one direction of deflection and to formv alinearly elongated cross-section whose. longest dimension is parallel to the other direction of deflection and which is located in the sagittal image plane which substantially coincides with the plane. of said screen for the said other direction of deflection, so that the cross section of said beam at the plane of said screen has a constant dimension in the said one direction of deflection for deflection throughout the entire scanned surface area of said screen.

2. Acathode-ray tube as claimed in claim 1, wherein said tube axis corresponds to the z axis of a rectangular co-ordinates system, said one direction of deflection corresponds to the if axis and said other direction of deflec tion corresponds to the x axis; said screen has strips of luminescent material; located thereon, the. length of said strips extending parallel to said x axis; and said coil system produces a deflecting field having an x component in said one direction of deflection, as measured in the plane x=O, determined by a power series of if of the form and an ij component in said other direction of deflection, as measured in the plane ij=0, determined by a power series of x of the form H at said end being negative, said term 211 orn:

having an absolute value which exceeds the absolute value of the term 21 HOI 3. A cathode-ray tube as claimed in claim 1, wherein said deflector coil system comprises a first pair of diametrically opposed coil halves surrounding the neck of said tube for deflection of said beam in said one direction of deflection and a second pair of diametrically opposed coil halves surrounding the neck of said tube and shifted substantially 90 with respect to said first pair of coil halves for deflection in said other direction of deflection,'said first and second pairs of coil halves being of such shape that the quantity --3+4 cos b 2 measured at the end of each pair of coil halves adjacent the screen of said tube is positive for said first pair of coil halves and negative for said second pair of coil halves and the absolute value of said quantity for said second pair of coil halves exceeds the absolute value of said quantity for said first pair of coil halves, where 1,0 is one fourth of the angle of the neck of said tube not embraced by a pair of coil halves and R is the radius of a pair of coil halves with respect to the axis of said tube.

4. A cathode-ray tube as claimed in claim 1, wherein said deflector coil system comprises first and second annular members surrounding the neck of said tube, said annular members being made of ferromagnetic material and said second member being located nearer to the screen of said tube than said first member; a first group of four coils Wound toroidally on said first member; a secand group of four coils wound toroidally on said second member; a third group of four coils wound toroidally on said first member; and a fourth group of four coils wound toroidally on said second member, said first and second groups of coils being operable to deflect said electron beam in said one direction of deflection and said third and fourth groups of coils being operable to deflect said beam in said other direction of deflection, the coils of each of said groups of coils being located in pairs on the annular member associated therewith, such that the coils of each pair are diametrically opposed and are adapted to have deflecting currents of opposite polarity flowing therein.

5. A cathode-ray tube as claimed in claim 4, wherein the radial angle formed by planes passing through each of the two pairs of coils in each of said groups of coils has a value larger than 60 for said first group of coils, smaller than 60 for said second group of coils, smaller than 60 for said third group of coils and larger than 60 for said fourth group of coils.

6. A cathode-ray tube as claimed in claim 4, wherein said deflector coil system further comprises circuit means adapted to couple said first and second groups of coils to a source of deflecting signal current, said circuit means being operable to permit the deflecting current in the coils of said first group of coils and the deflecting current in the coils of said second group of coils to be varied with respect to each other.

7. A cathode-ray tube as claimed in claim 6, wherein said circuit means comprises first means connecting the coils of said first group of coils in series, second means connecting the coils of said second group of coils in series, and third means adapted to connect the seriesconnected coils of said first group in parallel with the series-connected coils of said second group across the source of deflecting signal current, said third means including an asymmetrically adjustable impedance element having a terminal located at each end thereof and a tap located between said terminals, so that the impedance between one of said terminals and said tap and the impedance between the other of said terminals and said tap may be varied with respect to each other upon adjustment of said element, one of said terminals being connected to one of the free ends of said first group of coils and the other of said terminals being connected to one of the free ends of said second group of coils, said tap being adapted to be coupled to the source of deflecting current.

8. A cathode-ray tube as claimed in claim 6, wherein said circuit means comprises first means connecing the coils of said first group of coils in series, second means connecting the coils of said second group of coils in series, and third means adapted to connect the seriesconnected coils of said first group and the series-connected coils of said second group as adjacent legs in a bridge circuit across the source of deflecting signal current, said third means including an asymmetrically adjustable impedance element having a terminal located at each end thereof and a tap located between said terminals, so that the impedance between one of said terminals and said tap and the impedance between the other of said termi nals and said tap may be varied with respect to each other upon adjustment of said element, one of said terminals being connected to one of the free ends of said first group of coils and the other of said terminals being connected to one of the free ends of said second group of coils, said tap being connected to the other free end of both said first and second groups of coils, the terminals of said element being adapted to be coupled to the source of deflecting current.

References Cited in the file of this patent UNITED STATES PATENTS 2,196,838 Rogowski Apr. 9, 1940 2,252,441 Schlesinger Aug. 12, 1941 2,481,839 Goldsmith Sept. 13, 1949 

